The mass produced electrical outlet receptacle installed throughout homes and commercial buildings must be designed to meet Underwriters Laboratory (UL) standards. A new set of standards known as UL 498, which becomes effective in 1995, will impose even stricter design requirements on manufacturers. Receptacles are indeed rather simple electrical wiring devices, yet their design must be such as to withstand voltage surges of several thousand volts. They include internal line, neutral and ground circuit elements that must maintain a high degree of integrity over a long service life. In numerous installations, the receptacle must endure repeated plug insertions and withdrawals which are frequently executed in a haphazard manner. The internal contacts must be capable of accepting this abusive treatment without significant degradation of their low ohmic contact engagement with the line and neutral blades and ground pin of the plug. Moreover, the insulative plastic receptacle case must be sufficiently robust so as not to fracture when subjected to plug insertions and withdrawals that are not substantially normal to the receptacle face. The case must also provide requisite clearances between internal circuit elements to meet dielectric breakdown voltage requirements and have sufficient temperature stability to withstand "hot spots".
Continued integrity of the receptacle ground circuit elements is an important safety factor. If the ground circuit through the receptacle loses its normal low impedance characteristics, human shock potential due to the existence of a ground fault in a plug-connected load increases dramatically.
In addition to the above considerations, the receptacle must be designed to accommodate safe and convenient circuit installation by homeowners who are not electricians and have little experience and background in electrical matters. To this end, electrical receptacles are typically equipped with both binding head screw terminals and back-wire push-in (BWPI) terminals where the bared ends of solid wire line and neutral conductors are inserted through back openings in the receptacle case and into electrical contacting engagement with resilient prongs. Since BWPI terminations do not grip the conductor wire as securely as binding head screw terminations, they can be disturbed as the wired receptacle is mounted in a outlet box.